1. Field of the Invention
This invention relates to a cold rolling process for the manufacture of tubes by means of a Pilger mill. This mill comprises in a known way grooved rolls mounted in a mobile roll stand which makes a back and forth movement along the axis of the tube blank which is periodically advanced, the rolls being made to rotate by a known means. A mandrel is placed in the axis of the blank which is thus rolled between the rolls and the mandrel.
2. Description of the Prior Art
Modern Pilger mills of this type operate at high rolling rates which require moving the tube blank to be rolled for relatively short periods, at the very exact moments when the stand arrives at the end of its travel and when the rolls release the blank.
French Pat. No. 1,602,013 describes a Pilger mill having devices which impart periodic forward movements to the blank along its axis and rotation movements around this same axis when the rolls are at dead center and release the blank, at the end of their alternating back and forth travel.
The rotation movements are imparted to the blank by tube clamps placed on both sides of the roll stand. The periodic and discontinuous forward movements are imparted to the blank by an articulated pusher, mounted on a carriage which supports the rear of the blank and advances continuously in the direction of the stand.
As described in patent FR No. 1,602,013, this pusher, in the shape of a lever, is articulated around an axis. A cam controls the forward axial movement of the blank while imparting to the pusher a periodic rocking movement synchronized with the back and forth movement of the roll stand, when the stand is near dead center at the end of travel of one of its forward or return movements and when the rolls release the blank.
For clarity in the description, hereafter the input side of the blank to be rolled in the roll stand is called the upstream side and the output side of the tube from the stand after reduction of its section by rolling between the rolls is called the downstream side. The upstream-downstream movement of the stand and the rolls is called the forward pass and the downstream-upstream movement is called the return pass along the axis of the blank.
The rolling, according to the technique described in the patent FR No. 1,602,013, is usually done only during the forward pass, or from upstream to downstream from the rolls, the grooves of the rolls being designed to allow the free passage of the blank to be rolled for a short moment near the upstream dead center.
Thus, according to this use of the rolling mill, the rolls effectively work the metal by deforming it only during the forward pass of the stand. During the return pass, the rolls make only another pass over a product already rolled, without performing any deformation work.
FR No. 2,442,674 teaches, however, that it is advantageous to have the blank advance and turn only when it is released from the grip of the two rolls, when they are at the end of travel near the downstream dead center. Moreover, this patent teaches that the deformation of the blank is distributed between the forward and return passes of the rolls although the blank cannot be advanced near the upstream dead center.
FR No. 2,463,646 teaches that, to obtain the highest possible output, the blank is made to advance and turn near both the upstream and downstream dead centers of the forward and return movement of the rolls, the upstream advance being different from the downstream advance. This patent teaches that the maximum advance of the blank can, according to the circumstances, be done either upstream or downstream. The document, however, does not teach the criteria on which the definition of the upstream advance and the downstream advance must be based. Nor does it teach the means for making these advances, and further it does not teach the means that make it possible to assure, under satisfactory conditions, rolling during the return pass.
Moreover, given the heavy weights to be moved cyclically and which comprise, not only the roll stand but also the blank and its pusher, it is essential to have control means that exhibit at the same time a minimum of inertia, a high operating speed and great precision. Where there are none of these various means, it does not appear that cold rolling processes using a Pilger mill having a double advance of the blank has been hitherto developed.
Therefore, the possibility of developing a cold rolling process for tubes by means of a Pilger mill has been studied in which, to obtain maximum efficiency, the deformation work of the tube blank would be distributed suitably and, preferably equally between the forward pass and the return pass of the roll stand. The possibility of moving the blank cyclically downstream, near the upstream and downstream dead centers of the forward and return movement of the roll stand with speed and precision, while making possible its elongation upstream during the rolling during the return pass of the roll stand was also studied. Finally, a study was made to define the relation preferentially to be achieved between the advances that should be performed near the upstream dead center and downstream dead center to obtain during the rolling, during the forward pass more or less the same deformation work as during the rolling during the return pass.
The possibility was also studied of making a simple device that could be used on a Pilger mill, making it possible to make different advances near the upstream and downstream dead centers whose values are in a predetermined ratio and can be precisely reproduced.